To unlock the For Love of Persia trophy in Assassin's Creed Odyssey, you must first purchase the third episode of Legacy of the First Blade (or own the Season Pass) and download Episode 3.
There are seven members of the Order of Dominion to track down, but luckily four of them will be targets during the main quest anyway, leaving three more to find.
You will fight The Immortals very early in Episode 3 during the "Flight During Fire" quest.
You will meet Dimokrates during the "Ordering of the Kosmos" story quest.
You will meet Amorges during the main storyline quest of Episode 3.
Gergis the Herald:
You will meet Gergis as part of Episode 3's main quests. The first time you meet him, you will be given the choice if you would like to spare or kill him. The choice doesn't really matter. If you decide to spare his life, you'll be forced to fight him as part of the next quest.
Artazostre the Silence:
The clue for Artazostre the Silence can be found in Kaedes Cave, which can be found in the northeast of the Valley of Aristomenes, in Messenia. The cave will be marked on your map with the bandit camp symbol. Kill the enemies or sneak down in and enter the pit with a ladder. The clue to finding Artazostre can be found at the bottom.
You can hunt Artazostre the Silence down just to the southeast of the of Kaedes Cave.
Gaspar the Gatekeeper:
The clue for Gaspar the Gatekeeper can be found near Mount Ithome Fort, which is just north of the Bay of Hades in Messenia. Southwest of Mount Ithome, towards the marshes and near a road you will find a dead body that will have the clue.
You can hunt down Gaspar in the northeast of Messenia in a cave called Terror Gorge in the Mines of the Great Trench.
Pithias the Architect:
The clue for Pithias the Architect can be found in Pedasos Camp, which is south/southeast of the Bay of Hades in Messinia. In the camp, search the buildings for the clue next to a large leopard skin.
You can hunt down Pithias in the Aipeia, which is a coastal town in the Valley of Aristomenes region of Messinia.